The invention relates to a modulator for data transmission by frequency shift modulation comprising a first operational amplifier and an associated capacitor connected so as to form an integrating circuit, a bistable circuit having trigger hysteresis connected to the output of the first operational amplifier for producing a two-level voltage, a first resistor having one end connected to the output of the bistable circuit and the other end coupled to an input of the first operational amplifier so as to produce a triangular voltage at the output of the first operational amplifier, first logic means for applying a voltage equal or complementary to said two-level voltage, depending upon the value of the data signal, to one end of a second resistor whose other end is joined with said other end of the first resistor.
A modulator of this type may, for example, be used to implement 200-baud modems standardized by the CCITT. These modems comprise a modulator to transmit data over a "high" channel by using the frequencies F.sub.1 =1750 Hz+100 Hz and F.sub.2 =1750 Hz-100 Hz, and a modulator to transmit data over a "low" channel by using the frequencies F'.sub.1 =1080 Hz+100 Hz and F'.sub.2 =1080 Hz-100 Hz. The frequencies F.sub.o =1750 Hz and F'.sub.o =1080 Hz are called hereinafter the central frequencies of the high and the low channels.
U.S. Pat. No. 4,039,952 describes a modulator of the type set forth in the preamble by means of which it is possible to obtain a triangular-shaped or sawtooth signal whose frequency is shifted as a function of the data. However, this U.S. patent does not contemplate the case of a standardized 200-baud modem for data transmission over two channels with the same frequency shift around two different central frequencies. The ideal modulator would then be a universal modulator by means of which it is possible to pass from a "high"-channel modulator to a "low"-channel modulator and vice versa by means of a very simple change-over.